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By Root 847 0

getting increasingly dirty. Scientists working at the world’s highest atmospheric research station, 5,000 meters up near Everest Base Camp above the Khumbu glacier, have measured increasing levels of pollution streaming up the valley from densely populated regions far below,20 forming brown clouds sometimes thick enough to cause several degrees of local warming.21 This darker snow absorbs more heat from the sun—as much as double in some conditions, accelerating the melt of the glaciers and starting the melt season earlier in the year.22 The predicament of the Himalayan glaciers is not unique to Asia: In the European Alps, the continent’s diesel engines are thought to be depositing enough soot on glaciers to double their absorption of solar heat and rapidly accelerate the ongoing melt driven by global warming.

BLACK CARBON

While overall aerosols almost certainly have a profound cooling effect on the climate, one type of aerosol particle works strongly the other way. In fact, it may be the second largest contributor to global warming after CO2. This is the humble soot particle, known by atmospheric scientists as “black carbon.” Per unit of mass, black carbon is a million times more absorbent of heat than carbon dioxide,23 and even though it lasts in the atmosphere for only a few days, averaged out over a century it is—pound for pound—five hundred times more effective in warming the globe than CO2. By cleaning it up, we can not only improve respiratory health for human populations around the world, but we can also avoid a quarter to a third of ongoing global warming.24

Reducing black carbon emissions could immediately help stabilize both the polar ice caps and rapidly melting mountain glaciers. This is because, as I mentioned above, soot deposited on snow and ice makes the surface darker, accelerating melt rates. The importance of this can scarcely be overstated. As Stanford University’s Mark Jacobson puts it: “Controlling soot may be the only method of significantly slowing Arctic warming within the next two decades,” and thereby avoiding a runaway collapse of the Arctic sea-ice sheet.25 Because of the different timescales involved, dealing with black carbon is not a substitute for reducing greenhouse gas emissions, but it is a crucial accompaniment. Unfortunately, the issue has so far received very little attention either from campaigners, the media, or policymakers. This has to change, and quickly, because the world is missing an easy opportunity for a win–win environmental outcome that could relatively cheaply reduce the near-term warming of the globe at the same time as avoiding an estimated 1.5 million annual deaths per year from air pollution.

The challenge of black carbon should be an easy one to address, because nobody actually wants it. Unlike carbon dioxide, which is an unavoidable consequence of any use of fossil fuels for energy production, black carbon is only generated by incomplete combustion. We have all seen it, in the clouds of black smoke emitted by badly maintained diesel engines, or the sulfurous fumes of a home coal fire. The emission of black carbon is therefore easily avoidable by the improvement of combustion, the capture of particles, or the substitution of fuels. Importantly, black carbon also challenges the conventional narrative of global warming that sees rich countries as culprits and poor countries as innocent victims. Because it is typically produced by low-tech combustion, two thirds of the world’s soot comes from developing nations, making them nearly as responsible for day-to-day climate change as the industrialized world.26 Indeed, as much as a quarter of the global annual output of black carbon comes from just two countries: India and China.27

Some major sources of black carbon, like wildfires and other uncontrollable burning in open areas, we can do little about. But its production could be reduced by up to three-quarters by taking some relatively simple measures. In rich countries, soot comes mainly from diesel engines: This can be avoided by the mandatory sale of cleaner diesel